Many types of crash sensors have been proposed and used for determining if a crash is severe enough to require the deployment of a passive restraint system such as an air bag or seat belt tensioner.
Three types of sensors have been widely used to sense and initiate deployment of an air bag passive restraint system. These sensors include an air damped ball-in-tube sensor such as disclosed in Breed U.S. Pat. Nos. 3,974,350, 4,198,864, 4,284,863, 4,329,549 and 4,573,706, a spring mass sensor such as disclosed in Bell U.S. Pat. Nos. 4,116,132, 4,167,276 and an electronic sensor such as is part of the Mercedes air bag system. Each of these sensors has particular advantages and shortcomings which were discussed in detail in the patent cross referenced above.
The Use of tape or ribbon switch technology as a crush switch was also disclosed in the above cross-referenced patent. Further research has shown that an improvement of this particular implementation of the invention has significant advantages over some of the other implementations since the switch can be easily made long and narrow and it can be made to respond to bending. In the first case it can be designed to cover a significant distance across the vehicle which increases the probability that it will be struck by crushed material or bent as the crush zone propagates rearward in the vehicle during a crash. In the second case it can be made small and located to sense the fact that one part of the vehicle has moved relative to some other part or that the structure on which the sensor is mounted has deformed. The term "given position" of where the sensor should be located (rear boundary of the c"crush zone") is described in the '639 patent of Breed cited above at col. 6 line 57 et seq. and is also as to FIGS. 8-11 therein.
Other crush zone mounted crash sensors including crush switch designs where the width and height dimensions are comparable, must either be large and thus heavy, expensive and difficult to mount, or there is a possibility that the randomly shaped crushed material which forms the boundary of the crush zone will bridge the sensor resulting in late triggering. This crushed material frequently contains holes, wrinkles or folds or portions that may even be displaced or torn out during the crash with the result that it is difficult to guarantee that a particular small area where the sensor is mounted will be struck early in the crash.
A significant improvement results, therefore, if the sensor can stretch across more of the vehicle or if it can determine that there has been relative motion or deformation of a portion of the vehicle on which the sensor is mounted. The improved sensors described herein that are an object of this invention are small in height and thickness but can extend to whatever length is necessary to achieve a high probability of a sensor triggering on time in a crash. A short sensor can be used if it is carefully mounted onto portions of the vehicle which will cause the sensor to bend during a crash.
It has been found that conventional designs of tape or ribbon switches have the drawback that the force required to close the switch is very small compared with the forces which are normally present in automobile crashes. During routine maintenance of the vehicle, the normal tape switch may be damaged or otherwise made to close and remain closed, with the result that later, when the vehicle encounters a pot hole or other shock sufficient to cause the arming sensor to close, an inadvertent air bag deployment can result. Similarly, if the tape switch is mounted on the front of the radiator support, which is a preferred mounting locating for crush zone sensors, hail, heavy rain, stones or other debris from the road might impact the tape switch and cause a momentary closure or damage it. If this happens when the vehicle experiences a shock sufficient to cause the arming sensor to close, an inadvertent air bag deployment might also occur. The force typically required to close a tape switch is less than one pound whereas tens of thousands of pounds are required to stop a vehicle in a crash and local forces greatly in excess of 20 pounds are available to actuate a sensor during a crash.
The present invention seeks to eliminate these drawbacks through the use of a tape switch design which requires either a large force to actuate or a bending of the switch due to structural deformation as explained below.